This invention relates generally to light-emitting semiconductor devices, or light-emitting diodes (LEDs) according to common parlance, and particularly to those having a light-absorptive substrate and a transparent envelope. More particularly, the invention deals with an LED featuring provisions for improved efficiency and directivity.
The LED of typical conventional make comprises a light-generating semiconductor region grown on a substrate, an electrode to one of a pair of opposite major surfaces of the light-generating semiconductor region, and another electrode to the other major surface of the light-generating semiconductor region. The light-generating semiconductor region together with the substrate is encapsulated in a transparent envelope.
The substrate of the LED may be made from electrical insulators notably including sapphire, which is highly transparent, or from electrical conductors including silicon and compound semiconductors such as those based upon gallium arsenide (GaAs) or gallium phosphide (GaP). All these conductive are cheaper than sapphire and additionally advantageous over the insulators in making the substrate a path for the LED drive current. However, the conductors are far more absorptive of light than sapphire, causing a great deal of the light radiated from the light-generating semiconductor region toward the substrate to be thereby absorbed and wasted and so lessening the efficiency of the LED.
A remedy to this dilemma is suggested for example in Japanese Unexamined Patent Publication No. 2005-277372. It teaches use of a reflector between the light-generating semiconductor region and the substrate of light-absorptive material. The light radiated toward the substrate is saved by being reflected back toward the light-emitting surface of the LED.
This prior art device has proved to possess a weakness: Part of the light issuing from the light-generating semiconductor region is reflected by the envelope back toward the substrate to be thereby absorbed through its side surfaces. As the LED has developed more and more in recent years for less energy consumption and greater optical output, such light absorption by the substrate presents now a hitch not to be overlooked for still higher LED efficiency. The Japanese patent publication cited hereinabove is silent on how to defeat this problem
Efficiency aside, the LED is required to possess “directional uniformity,” by which is meant a measure of evenness of light intensity in a given zone of illumination. This property is of particular significance in use of the LED as illuminator or the like. The patent publication above discloses no measures for directional uniformity, either.
Japanese Unexamined Patent Publication No. 2004-55816 makes a different approach to how to improve the efficiency of an LED incorporating a sapphire substrate. This second prior art device has its light-generating semiconductor region so formed on the surface of the substrate as to leave an annular marginal part of the surface unoccupied. An annular slit is formed in this marginal part of the substrate surface. The side of the light-generating semiconductor region, as well as the surfaces bounding the annular slit, is covered with a reflective film in order to make possible the emission of the light that has been radiated laterally of the light-generating semiconductor region.
It is submitted that the second prior art device is not anticipatory of the instant invention. The former is expressly designed for enhancement of efficiency through reduction of lateral light diffusion from the light-generating semiconductor region and substrate. The present invention presupposes use of a substrate of light-absorptive material and aims at prevention of the light reflected inwardly by the LED envelope from being absorbed by the substrate.